Solder leveller

ABSTRACT

A solder leveller for tinning exposed metal on printed circuit boards comprises a solder bath and paired rollers therein for carrying a board in a curvilinear path downwardly into, through and upwardly out of the bath. At least one pair of rollers having a board engaging nip within the bath is driven. The bath includes means for providing a solder-oxidation inhibiting oil over the solder.

THE FIELD OF THE INVENTION

The present invention relates to a solder leveller.

THE BACKGROUND OF THE INVENTION

A solder leveller is apparatus for tinning exposed metal--normallycopper--on printed circuit boards, and similar elements--particularlyfor supporting electronic components--hereinafter referred to simply as"boards", with solder preparatory to connection of components thereto.Generally a solder leveller comprises a bath of molten solder into whicha board is lowered vertically for tinning with solder and means forleveling solder deposited on the board as it si withdrawn from the bath.Such a leveller is the subject of British Patent No. 2151528 andequivalent U.S. Pat. No. 4,599,966, both in the name of the presentapplicant.

Such conventional solder levellers operate on a one-at-a-time basis,inserting boards into a deep, narrow soilder bath one-at-a-time fromabove. This is an essentially slow process, although the speed ofwithdrawal from the solder is relatively high in terms of speed oftravel of boards along conveyors through pre- and post- treatmentstages, up-and down-stream of the leveller. A feature of a verticalacting solder leveller is that the lower part of the board remains inthe solder for longer than the upper part. This can result in formationof a thicker copper/tin intermetallic compound layer on the lower partto which it can be difficult to satisfactorily solder components. Such athicker layer can cause a board to "age" more rapidly, aggravatingsolder-ability problems.

Intermetallic compounds can be both of the Cu₆ Sn₅ type and of the Cu₃Sn type. The latter is particularly problematic and more prone toformation where the board experiences longer dwell times in hottersolder.

Equipment for continuous soldering treatment is known in various forms,in particular for soldering components to pre-tinned boards. "Dragsoldering" involves the dragging of a board with its components acrossthe surface of a bath of molten solder, see for instance U.S. Pat.Specification No. 4,284,225 (EMP Ag). "Wave Soldering" involves passinga board across the top of a wave created, in a bath of molten solder,see for instance British Patent Specification No. 2,117,690A. (ZevatronGmbH).

Both drag and wave soldering have been adapted for continuous tinning.U.S. Pat. Specification No. 4,277,518 (Gyrex Corp.) describes the use ofa continuous conveyor forced down into molten solder by a fixed upperbed to drag a board through the solder. The board tends to float upagainst the conveyor which causes frictional engagement of the boardwith the conveyor. The speed of forward operation is limited bypotential slippage between the board and the conveyor.

U.S. Pat. Specification No. 4,465,014 (Siemens Ag) describes movement ofa board through a solder wave so that solder wells on to the top surfaceof the board. Speed of operation is limited by time available for heattransfer from the solder to the board.

The object of the present invention is to provide an improved solderleveller capable of operating fast enough for effective operation of thelevelling means and yet able to heat the board quickly in a compactsolder bath with a short dwell time in the bath to minimize theformation of an intermetallic compound layer.

Another object of the invention is to provide a solder levellerincluding means by which a board is rapidly heated to the temperature atwhich exposed copper on the board is wetted by solder.

Another object of the invention is to provide a solder levelleroperating substantially horizontally to provide even treatment of theentire board and in which the board is moved fast enough for successfulair knife levelling.

THE INVENTION

A solder leveller according to the invention includes:

curved-path defining rollers arranged across the bath to define a curvedportion of a board path through the solder leveller;

at least two of the curved-path defining rollers comprising a pair ofsuperimposed, biased together, board-gripping rollers having a niptherebetween beneath the free surface of the molten solder at which theboard is gripped for drive;

means for rotational drive of at least one of the said pair of theboard-gripping rollers;

the board path having:

an upstream portion above the free surface of the molten solder leadingto,

the curved, board-heating and tinning portion below the surface of thesolder, including the nips of the or each said pair of board-grippingrollers, at which the board is gripped and leading to,

a downstream portion above the free surface of the solder to thelevelling means.

Normally the board-gripping rollers will be metallic and there willusually be more than one pair of them. The curved-path defining rollersmay also include guide rollers provided singly and/or in pairs withinand/or without the solder bath up- and/or down-stream of theboard-gripping rollers.

In operation, a board approaches the surface of the molten solder alongthe up-stream portion of the board path and is moved below the surfaceof the solder. Here it is firmly grasped by the first of the pairs ofboard-gripping rollers and moved forwards at their brisk speed ofrotation. In being gripped, the board has heat transferred to it byconduction from the rollers. It passes through the nips of thesuccessive pairs of rollers. There are preferably three. As soon as itis heated to the melting point of solder, the copper contact pads etc.on the board are wetted by the solder and the board is thus tinned.

Because, in passing through the bath, the board is substantiallyhorizontal--whilst first having a slight inclination in one directionand then having a slight inclination in the other direction--and becauseof the action of the board-gripping rollers in moving solder towards andaway from the board, any air pockets in through-plated board holes arepositively displaced allowing the plating of these holes to be tinned.

Having been passed around the path's curve, the board emerges from thesolder at a speed suitable for levelling of the solder in the levellingmeans, preferably so called air knives.

The pairs of board-gripping rollers may be biased together in a varietyof manners. Since most materials are buoyant in molten solder, the upperof each pair of these rollers may be journalled in a translationallyfixed manner, whilst the lower roller is so journalled as to be free tofloat up against the upper roller. Alternatively, the lower roller maybe fixed and the upper roller biased downwards, either by spring forceand/or by its own weight.

The board-gripping rollers may be journalled in bearings attached to orguided on side walls of the solder bath. However, in the preferredembodiment a pair of separate roller support beams is provided extendinglongitudinally of the bath. Preferably the lower rollers are journalledin graphite bearing blocks held on the beams, whilst the upper rollersare journalled in graphite bearings able to move vertically on the beam,though biased down by their weight and bias springs onto the lowerrollers.

Each upper roller may be constrained to move vertically and nothorizontally by engagement of its bearings with the beams. However, inthe preferred embodiment, the central upper roller alone is soconstrained, whilst the upper rollers adjacent up- and down-stream alongthe board path are linked to the central roller by links interconnectingadjacent upper rollers' bearings.

Firm gripping of a board by the board-gripping rollers is important notonly to drive the board along the board path, but also for heat transferby conduction from the rollers to the board. Accordingly, in thepreferred embodiment, board-gripping rollers have a large diameter andhence thermal capacity. Further guide rollers where provided willnormally be of smaller diameter. In the preferred embodiment, guiderollers are provided both in pairs to grip the board though lessstrongly than the main rollers and as a single guide roller. The guiderollers direct the board in and out of the bath along the desired boardpath.

The board path is curved during that part of the board path of passingbelow the surface of the solder. Despite printed circuit boards beingrigid at room temperature, curvature of them to pass along the boardpath is feasible because at the temperature of molten solder the boardsbecome relatively flexible and because the radius of curvature of thepath is comparatively large. Either of the upper or lower board-grippingrollers can be driven. However, in the preferred embodiment, the axes ofthe upper rollers are arranged above the surface of the solder, and itis convenient to drive the upper rollers directly--with the lowerrollers being driven by directional contact between the pairs ofrollers.

Solder circulation is important in a solder bath, since solder is a poorconductor of heat, as compared with most metals. Whilst a solder pumpcould be provided, it is anticipated that rotation of the totallyimmersed lower rollers may provide adequate circulation. However,further circulation is preferably provided for by means of at least oneof the lower rollers having an impeller for moving solder axiallythrough the roller.

To help understanding of the invention, a specific embodiment thereofwill be described by way of example with reference to the accompanyingdrawings.

THE DRAWINGS

FIG. 1 is a side view of a solder leveller according to the invention;

FIG. 2 is a central cross-sectional side view to a larger scale on theline II--II in FIG. 3,

FIG. 3 is cross-sectional plan view on the line III--III in FIG. 2 fromwhich certain components have been omitted for the sake of clarity;

FIG. 4 is a cross-sectional end view on the line IV--IV in FIG. 3.

THE PREFERRED EMBODIMENT

Referring to the drawings, FIG. 1 shows a leveller of the inventionhaving a solder bath 1 mounted on a frame-work 2 having wheels 3 engagedon rails 4 extending transversely of the solder bath. Solder levellingair knives 5 are mounted on the bath at its downstream end. An upstreamconveyor 6 is arranged for transporting boards to be tinned to the bathand a downstream 7 conveyor is arranged for transporting away tinnedboards. The conveyors 6, 7 are arranged at angles α, β--both typically13° --of entry and exit of the boards to and from the bath 1. Theconveyors 6,7 are driven from a motor 44--described below --insynchronism with board-gripping rollers--also described below. Upstreamof the feed-in conveyor 6 are arranged, but not illustrated,pre-treatment stations including cleaning, flux and preheating stations;downstream of the feed-out conveyor 7, again not illustrated, arepost-treatment stations including cooling and washing stations. Thesepre- and post-treatment stations feed boards at typical speed of 1.5 to7.5 m/min.

Also shown in FIG. 1 is the upper, cover portion 8 of a cast, rollersupport beam described in more detail below and non-heating portions 9of heating elements extending in the solder bath.

Referring now to the other Figures, the solder bath 1 has three pair ofupper and lower board-gripping rollers 10_(u), 10₁ ; 11_(u), 11₁ ;12_(u), 12₁, extending transversely of it. The rollers can be of metalwhich is not normally wetted by solder for instance stainless steel,nickel or titanium. However, it is preferably of wettable--though solderinsoluble--metal for instance mild steel to provide even solder contactwith the board. These rollers are journalled via end shafts 14 in upperand lower graphite bearings 13_(u), 13₁ carried on roller support beams15, extending along the bath 1, one on one side of the rollers and theother on the other side. The lower bearings 13₁ are of rectangularcross-section and are held captive in downwardly open complementaryseats 16 by straps 17 to translationally fix the lower rollers. Itshould be noted that although the straps 17 support the weight of thelower rollers when the solder bath is empty, when it is full of moltensolder the lower rollers 10₁, 11₁, 12₁, tend to float so that they arelocated directly by engagement of the bearings 13₁ in their seats 16.

The upper graphite bearings 13_(u) each comprise a pair of flangedannular bushes 18 accommodated within a stainless steel, cylindricalsleeve 19. The sleeves 19 of the central, upper roller 11_(u) arelocated in vertically-extending, vertically extending bearing horns 20in the beams 15. The horns are of such height that the upper roller11_(u) can drop onto the lower roller 11₁ or be lifted slightly from itby the thickness of a typical board B. A pair of links 21, having eyes22 engaging around the sleeves 19 for the roller 11_(u), extendrespectively up- and down-stream of the bath to the rollers 10_(u),12_(u), where further eyes 23 at the other ends of the links 21 engagearound the sleeves 19 for these rollers 10_(u), 12_(u). The beam 15 andits upper cover portion 8 have a cavity 24 at each bearing 13_(u) withinwardly-directed side faces 25 laterally captivating the links 21--andfurther links 26, 27 to be described--via their eyes 22, 23. Thus thecentre links 21 are able to swing and move up and down with the upperrollers 10_(u), 11_(u), 12_(u), whilst the centre distances of therollers are kept constant as they move to allow the board B to passbetween the pairs of rollers.

The horns 20' at the sleeves 19 for the rollers 10_(u), 12_(u) arerelieved to allow for slight movement of these rollers axially of thebath as the links 21 fore-shorten.

The upper rollers 10_(u), 11_(u), 12_(u) are urged towards the lowerrollers 10₁, 11₁, 12₁ by springs 28 acting on the eyes 22, 23 and housedin bores 29 in the upper beam portion 8. Each spring 28 is pre-loaded byan adjustable eccentric 30 accommodated on the beam and a pin 31 passingthrough the spring, whereby the degree of bias of each upper rollertowards its lower roller can be adjusted to suit the thickness of theboard B.

The beams 15 are secured to the solder bath 1 at weirs 32, 33 describedbelow, thus they locate the rollers laterally of the bath by contact ofthrust spacers 34 with the inner, opposed faces 35 of the beams and theends of the rollers. The bearings 13_(u), comprised of the bushes 18,are located on the upper rollers' shafts 14 by thrust collars 36 securedto the shafts.

On the upstream side of the board-gripping rollers 10_(u), 10₁, 11_(u),11₁, 12_(u), 12₁, a lower idling guide roller 37 is provided, journalledin the same manner as the lower gripping rollers. Upstream of this is apair of guide rollers 38_(u), 38₁ journalled in the same manner as themain board-gripping rollers. Links 26 extending from the rollers 10_(u)have eyes 39 adjacent to eyes 23 and an enlarged eye 40 at the roller38_(u) performing the function of the sleeve 19 in accommodatinggraphite bushes 18'. Similarly on the down-stream side a pair of guide41_(u) ; 41₁ is provided in like manner to the rollers 38_(u) ; 38₁,with the upper roller 41_(u) linked to the roller 12_(u) by links 27.Thus all the upper rollers can be removed for service as one linkedtogether unit.

To one side of the bath, the upper rollers' shafts 14 are extended andcarry sprockets 42. Those on the shafts of the rollers 10_(u), 11_(u),12_(u) are double and those on the shafts of the rollers 28_(u), 41_(u)are single. Respective drive chains 43 extend between each adjacent pairof sprockets 42, drivingly linking the adjacent rollers, whereby all theupper rollers are ganged together. A motor 44 drives the rollers via adog clutch 45. Except for the idler roller 37, the lower rollers 38₁,10₁, 11₁, 12₁, 41₁ are all frictionally driven by their upper rollersdue to the upper rollers being biased down onto the lower rollers. Toguard against snatch of a board B and consequent damage to it, as itenters the first pair of rollers 38_(u), 38₁ and is accelerated by them,a torque limiting clutch 46 is provided between the upper rollers'sprocket 42 and its shaft 14. The dog clutch 45 is provided to enablethe bath to be disconnected from the motor 44 and withdrawn on its rails4 for service.

The bath 1 is constructed of mild steel plate and supported on a chassis47 itself supported on the framework 2. Along each side of the bath,channels 48 are provided inboard of which sides 49 extend up and dropdown into a sump 50 centrally of the bath. Solder heating elements 51are arranged across the bath in the sump 50 and have non-heatingportions 9 in which their resistance wire is not coiled--looping overthe sides 49 and out through the bottom of the channels 48.

At each end of the bath the liquid height control weirs 32, 33 areprovided. Upstream of the upstream weir 32 a further small sump 52 isprovided. In use, solder 53 is added as required to keep the freesurface 54 of the solder just below the level of the weirs.Anti-oxidation oil 55 is pumped from the further sump 52 onto the solder53 to flow back over the weir 32 and keep the solder covered.

In use of the solder leveller, the sump is filled with solder 53 whichis kept molten by the heating elements 51. During idling, the motor 44keeps the rollers turning at slow speed typically 5 rpm, this circulatesthe solder, partially by the solder being dragged around by theboard-gripping rollers' peripheries and partly by the lower rollersbeing hollow and fitted with impellers 56 for moving the solder in onedirection 57 through the roller 11₁ and back in the other direction 58in the rollers 10₁, 12₁. During operation the motor speed is variedbetween a board-gripping roller speed matching the pre- andpost-treatment station speeds and a processing speed. When a board isdetected as approaching down the feed-in conveyor 6 the motor isaccelerated to provide a processing roller peripheral speed of theregion of 50 m/min, given by roller rotation at 125 rpm for 125 mmdiameter rollers. The board B on reaching the rollers 38, the lower ofwhich runs in the oil 55, is accelerated and fed through the freesurface 54 of the solder along an upstream portion 59 of a board paththrough the leveller. It is guided by the roller 37 and passes into thenip of the rollers 10. In passing along a curved portion 60 of the paththrough the nips of the three pairs of board-gripping rollers 10_(u),10₁ ; 11_(u), 11₁ ; 12_(u), 12₁, it is heated from an initialtemperature in the region 50-150° C. to just above the eutectic point ofthe solder i.e. 183° C., by contact with the solder and moreimportantly, contact with the rollers which conduct heat to the board.The temperature of the solder is controlled in the region 220-260° C.;although the lower end of the range is preferred to inhibit theformation of inter-metallic compounds. In this respect it can be notedthat parts of the board remain in the solder for equal lengths of time.On reaching the eutectic temperature the board's contact pads wet andare tinned. The board is fed out from the solder on a downstream pathportion 61 and through the air knives 5 at the roller speed foreffective levelling. As soon as the board clears the air knives, therollers, together with the up- and down-stream conveyors, are returnedto the speed of the pre- and post-treatment stations to await the nextboard.

Typical dimensions of the solder leveller are as follows:

    ______________________________________                                        Board-Gripping roller diameter                                                                         125 mm                                               Board-Gripping roller length                                                                           650 mm                                               Bath length between weirs                                                                              600 mm                                               Radius of curve path     800 mm                                               Guide roller diameter     50 mm.                                              ______________________________________                                    

The invention is not intended to be restricted to the details of theabove described embodiment. The leveller described is appropriate forprocessing boards which are up to 610 mm wide and of indefinite lengthover 290 mm. For other size of boards, the bath dimensions and theboard-gripping roller number and dimensions may vary. The specifiedoperating speeds and temperatures may vary. The overriding requirementis of course that the boards have been heated to the eutectic point andbeen wetted by the solder on emerging from the solder. The air knivesmay be constructed to be rigid with the bath or may be separate andspaced from the bath along the downstream portion of the board path.

I claim:
 1. A solder leveller having a bath for containing molten solderwith a free surface, said solder leveller including:curved-path definingrollers arranged across said bath to define a curved portion of a boardpath through said solder leveller; at least two of said curved-pathdefining rollers comprising a pair of superimposed, biased together,board-gripping rollers having a nip therebetween beneath said freesurface of said molten solder at which the board (B) is gripped fordrive; means for rotational drive of at least one of said pair ofboard-gripping rollers; means for levelling solder deposited on a board(B) as it leaves said bath; and means for providing a layer ofsolder-oxidation-inhibiting oil over said molten solder in said bath,said means including a sump from which said oil is pumped onto saidmolten solder and a weir over which said oil flows back to said sump toregulate depth of said oil over said molten solder, said nip of saidpair of said board-gripping rollers being below the level of said weir;said board path having:an upstream portion above the free surface of themolten solder leading to, the curved, board-heating and tinning portionbelow the surface of the solder, including said nips of the or each saidpair of board-gripping rollers at which the board is gripped and leadingto, a downstream portion above the free surface of the solder leading tosaid levelling means.
 2. A solder leveller according to claim 1, whereinthe curved-path defining rollers comprise, in addition to saidboard-gripping rollers, guide rollers provided singly and/or in pairswithin and/or without the solder bath.
 3. A solder leveller according toclaim 1, wherein said oil sump is upstream of said solder bath and oneof said guide rollers runs in said oil sump.
 4. A solder levelleraccording to claim 1, wherein a plurality of pairs of saidboard-gripping rollers is provided, said solder leveller includingrespective bearings fixed with respect to said bath for one of each pairof said board-gripping rollers and respective bearings translationallymovable with respect to said bath for the other of each pair of saidboard-gripping rollers.
 5. A solder leveller according to claim 4,wherein said bearings are of graphite.
 6. A solder leveller according toclaim 4, wherein said lower ones of the pairs of board-gripping rollershave said fixed bearings and said upper ones of said pairs ofboard-gripping rollers have said movable bearings, and said solderleveller includes means for urging said translationally movable bearingstowards said fixed bearings, whereby said board-gripping rollers in eachpair are biased together.
 7. A solder leveller according to claim 6,wherein said urging means comprise a respective spring acting at one endthereof on each said translationally movable bearing.
 8. A solderleveller according to claim 7, including adjustment means acting on theother end of each said spring, whereby the degree of bias of the or eachsaid upper board-gripping roller towards the respective one of saidlower board-gripping rollers is adjustable to suit the thickness of theboard (B) to be passed along said curved portion of said board path. 9.A solder leveller according to claim 4, including a respective rollersupport beam at each end of said board-gripping rollers on which bothsaid fixed and said translationally movable bearings at the respectiveends of said board-gripping rollers are located, said beams extendinglongitudinally of and being fixed with respect to said bath.
 10. Asolder leveller according to claim 9, including respective inter-bearinglinks linking adjacent pairs of said movable, upper,board-gripping-roller bearings at each end of said upper board-grippingrollers, whereby separation of adjacent upper ones of said roller alongsaid curved portion of said board path is determined.
 11. A solderleveller according to claim 10, wherein a central one of said upperboard-gripping rollers is constrained against horizontal movement byengagement of its said bearings with said beams.
 12. A solder levelleraccording to claim 10, including drive sprockets carried on each saidupper board-gripping roller and drive chains extending between saiddrive sprockets of adjacent rollers for ganging together rotationaldrive of said upper board-gripping rollers and driving said board (B)through said solder.
 13. A solder leveller according to claim 1,including a drive motor and a drive connection from said motor to saidupper board-gripping roller(s).
 14. A solder leveller according to claim13, including a support framework for said solder bath, support railsextending transversely of said board path, wheels journalled on saidframework and running on said support rails for lateral withdrawal ofsaid solder bath, and a dog clutch incorporated in said drive connectionwhereby said board-gripping rollers are drivingly disconnected from saidmotor on withdrawal of said solder bath.
 15. A solder leveller accordingto claim 2, including at least three pairs of said board-grippingrollers and two pairs of guide rollers, one upstream of saidboard-gripping rollers and one downstream thereof.
 16. A solder levelleraccording to claim 15, including a drive connection between said pair ofboard-gripping rollers having said rotational drive means, said driveconnection including a torque limiting clutch between said drivenupstream guide roller and its drive sprocket.
 17. A solder levelleraccording to claim 1, wherein at least one of said lower board-grippingrollers is hollow, open at both ends and provided with an impeller forcirculating said molten solder therethrough.
 18. A solder levelleraccording to claim 1, wherein said solder bath includes a second weirfor controlling the height of the free surface of saidsolder-oxidation-inhibiting oil above the free surface of said moltensolder in said bath.
 19. A solder leveller according to claim 18,including a pair of roller support beams on which said curved-pathdefining rollers are journalled, said roller support beams beingsupported at opposite ends thereof on said weirs.
 20. A solder levelleraccording to claim 1, including electric solder heat elements extendingin said bath across its bottom.
 21. A solder leveller comprising:a bathfor molten solder, air knives at a downstream end of said bath, threepairs of board-gripping rollers arranged across said bath to define acurved board path portion below a free surface of said solder in saidbath, one pair of guide rollers upstream of said board-gripping rollersfor guiding a printed circuit board (B) into the nip of an upstream oneof said board-gripping rollers, two beams one extending along one sideof said bath and the other along the other side for supporting saidrollers, lower bearings fixed to said beams for journalling said lowerones of said pairs of board-gripping and guide rollers, upper bearingsmovable up and down on said beams for journalling upper ones of saidpairs of board-gripping and guide rollers, springs acting on said upperbearings for urging said upper rollers into board-gripping contact withsaid lower rollers, a drive motor for driving said upper rollers inrotation, and sprockets and chains for driving said upper rollers insynchronism.
 22. A solder leveller according to claim 21, includinganother pair of guide rollers downstream of said board-gripping rollersfor guiding the board (B) through said air knives.
 23. A solder levelleraccording to claim 21, including:an oil sump upstream of said solderbath, the lower one of said upstream pair of guide rollers running insaid oil sump, and a pair of weirs for controlling the height of thefree surface of solder-oxidation-inhibiting oil above the free surfaceof said molten solder in said bath, the nip of said board-grippingrollers being below the level of said weirs.
 24. A solder levelleraccording to claim 21, including:support rails extending transversely ofsaid board path; a support framework for said solder bath; wheelsjournalled on said framework and running on said support rails forlateral withdrawal of said solder bath; a drive motor for saidboard-gripping rollers fixed with respect to said support rails; and adrive connection including a dog clutch between said drive motor andsaid board-gripping rollers, whereby said board-gripping rollers aredrivingly disconnected from said motor on withdrawal of said solderbath.
 25. A solder leveller as claimed in claim 21, including a furtherdrive connection including a torque limiting clutch between saidupstream pair of guide rollers and said board-gripping rollers.